The invention relates to a method and to a transmitter arrangement and a receiver arrangement for probing drill holes.
Such a method and arrangements for carrying out the method for probing drill holes are known from EP-C-0 494 130. A theoretical exposition of the general measuring method is given in the Geological Survey of Canada Paper 85-27 (Toronto 1986), pp. 79-88, by Pantze et al.
Briefly, such methods involve passing a low-frequency alternating current from a transmitter unit through a ground carried loop whose magnetic field magnetises underlying minerals or generates induction currents therein. A receiver carried by a probe in a drill hole determines direction, amplitude and phase in respect of the sum of the primary and secondary induced magnetic fields that have the same frequency as the loop current.
In all known systems (e.g. in addition to systems taught by EP patents GB-A-1 467 943, and GB-A-2 148 012) detection is effected in one way or another phase locked to the frequency of the transmitter, wherewith the signal delivered by the probe is collected through an electric cable. Such systems have several drawbacks. All is well provided that the drill holes are stable and steep. However, the rock is often so weak as to risk a cable-suspended probe getting jammed in the hole. It is also difficult to force a probe down into a drill hole against any relatively firm resistance, for instance in the case of very flat or superficial holes or when the holes even extend upwards or are filled with sludge.
An object of the present invention is to provide a drill hole measuring system of the aforesaid kind with which the drawbacks associated with cable suspended probes are eliminated. Another object is to provide a system which is more robust mechanically, which can be applied more simply, and with which actual sampling can be handled by drilling personnel that have a modicum of training, while allowing the data collected to be evaluated by prospecting experts under laboratory conditions.
These and other objects of the invention and advantages afforded thereby are realized by means of a method and a transmitter arrangement and a receiver arrangement of the aforesaid kind that have the special characteristic features set forth in respective independent claims, as will be apparent from the following description. Further advantageous embodiments of the invention will be apparent from the dependent claims.
Because there is no longer any electric connection between the transmitter and the receiver when practicing the present invention, it is necessary to measure time accurately. With respect to the transmitter, there can be used very precise time standards transmitted in radio connections. Although precision transmissions occur as terrestrial radio signals that derive from atomic clocks, the use of GNSS signals (acronym for Global Navigation Satellite System), e.g. belonging to the GPS system is preferred at present.
In accordance with one advantageous embodiment of the invention, orientation of the probe in relation to the vertical is determined with the aid of a series of accelerometers instead of a gimbal and pendulum as in the case of known techniques, said accelerometers determining the direction of the vertical in relation to three Cartesian co-ordinate axes of the probe, each of said axes being defined by a respective accelerometer.
According to another advantageous embodiment of the invention, the probe is supplemented with a magnetic three-component sensor (flux gate) which measures the terrestrial magnetic field. Because the terrestrial magnetic field has a direction that differs from the vertical, it is possible to fully determine the position of alignment of the probe from this measurement in combination with the measuring result obtained with the accelerometers.
By registering the local terrestrial magnetic field at the same time, it is possible to interpret the local magnetic susceptibility in the rock and to judge where the terrestrial magnetic field is so undisturbed by local anomalies as to enable it to be used to accurately determine direction in relation to geographic north.
The strength of the primary field can also be measured by the magnetometer, by sending a constant direct current in the transmitter loop over a period of some seconds. (The duration of this current is adapted to the conductivity of the rock under investigation.) The primary field is determined directly, by measuring the magnetic field with the magnetic sensors with and without said direct current, regardless of electrical conductors in the bedrock. Since the magnetometer sensors are not sensitive to alternating current fields, the AC-field may be constantly present. A change of 10 nT is obtained in the field with some tens of amperes in the loop, in the case of normal transmitter lay-outs and drill hole depths.
The probe will conveniently be compatible with the modularized, connectable pipe sections (the drilling rod) used in the actual drilling operation and will thus include the same type of connector as said sections and the drill bit attached to the rod. When drilling of a hole is completed, the drilling bit is lifted up and the pipe sections successively dismantled. A drill bit may optionally be fastened to the bottom part of the probe and the pipe sections fitted together exactly as while drilling. The presence of a drill bit at the end of the probe avoids the probe becoming jammed and lost, this risk being far less than in the case of a line-suspended probe. Weak zones can be xe2x80x9cdrilledxe2x80x9d through in particular, since the probe is able to withstand such drilling action under such circumstances. Because the probe is attached to a drilling rod, it can be moved down into oblique drill holes and even in horizontal or upwardly sloping parts of a drill hole.
When lowering the probe, the foreman will note on a keypad belonging to the transmitter part, or on a separate hand-held unit, the instants at which the drill rod joints enter the drill hole, these notations then being used in the evaluation process. The probe is suitably allowed to carry out registrations at regular time intervals, e.g. every fifteen seconds. This can then be combined with the probe-lowering protocol. The measurements are suitably repeated at the same locations or positions when lifting up the probe.
In accordance with one preferred embodiment of the invention, the operating personnel have a hand-held computer which includes a clock function in which time and depth are stored. A suitable distance along the drill hole between said measuring points may be 3 m, corresponding to the length between the joints in the drill pipe. Upon completion of the measuring process and return of the probe to the surface, the probe and the hand-held computer are connected and caused to communicate with one another, wherewith the computer calls the probe for each registered time point with depth and receives the recorded values at this time point, such that all data will be stored in the computer and later sent for processing.
It is suitable to read into the probe memory all available data at regular time intervals, e.g. every fifteen seconds, so as to ensure that at least one measurement for each level can be fed into the computer.
This simple procedure, involving and utilizing the same equipment as that used for drilling purposes, enables the measuring operation to be carried out as a matter of routine by the same personnel as those drilling the hole. Traveling time for special personnel can be saved by laying-out the transmitter loop beforehand (perhaps for use with several drill holes) and drill holes that are completed at night-time can be measured with the minimum of time loss.